1. Field of the Invention
The present invention relates to a solar cell and a production process therefor. More particularly, the present invention relates to a space-use solar cell to be on board of an artificial satellite and a process for producing such solar cells.
2. Description of Related Art
The construction of conventional solar cells of this kind, especially, conventional common solar cells for space use, is described with reference to accompanying figures.
FIG. 7 shows a perspective view illustrating the construction of a conventional common high-efficiency solar cell for space use. The solar cell is composed of a crystalline substrate 1 of a first conductivity type, e.g., of a P-type silicon substrate, a region 2 of a second conductivity type, e.g., of N-type, formed on a light-receptive face (upper surface) of the substrate 1, a P.sup.+ -type region 3 formed on a lower surface of the substrate 1 for a back surface field (BSF) effect, an insulating film 4 on an N-type region side which is formed on a upper surface of the N-type region 2, an insulating film 5 on a P.sup.+ -type region side which is formed on a lower surface of the P.sup.+ -type region 3, an N-electrode 6 provided on a surface of the N-type region 2, a P-electrode 7 covering almost the entire lower surface of the P.sup.+ -type-region-side insulating film 5, an anti-reflection film 8 covering almost the entire upper surface of the N-type-region-side insulating film 4 and the like. On the light-receptive face, a reflection-reducing surface structure 9 having a large number of projections and depressions, for example in the shape of inverted-pyramid, is provided for reducing reflection.
FIG. 8 is a perspective view illustrating the construction of a simplified common solar cell for space use. The light-receptive face thereof has a substantially flat structure 10.
The high-efficiency solar cell for space use is produced, for example, by a process as shown in schematic sectional views of FIGS. 9(a) to 9(g) illustrating a production process.
For example, a semiconductor substrate 12 which has been cut from an ingot of monocrystalline silicon in the form of a wafer is usually about 300 .mu.m thick at the thinnest. This semiconductor substrate is etched with an acid or alkaline solution or polished to a thickness d1 of 50 .mu.m to 200 .mu.m, thereby to obtain a substrate 1 (see FIG. 9(a)). This thickness is determined depending on complexity of later production steps and desired solar cell characteristics.
Generally, the thinner the substrate is, the less the characteristics of solar cells deteriorate by being exposed to radiation such as cosmic rays. On the other hand, the thinner the substrate is, the more difficult the production is. The substrate is required to be strong to a certain degree, that is, must be somewhat thick, especially for producing a solar cell which has a high photoelectric conversion efficiency and thus has a complicated structure.
For example, in the case of a monocrystalline silicon solar cell as shown in FIG. 8 which requires relatively simple production steps later, the production thereof is easy and the thickness d1 of the substrate may be selected within the range of about 30 .mu.m to about 300 .mu.m depending on the cell characteristics. However, since the construction of the cell is simple, the photoelectric conversion efficiency is low. The initial photoelectric efficiency with regard to an AM0 light source before exposure to radiation is about 14%.
In contrast, in the case of a high-efficiency solar cell as shown in FIG. 7 which has a complicated construction, an initial photoelectric efficiency of about 17% can be obtained. However, this type of solar cell is more difficult to produce than the cell of FIG. 8, and when a thin substrate is used, the substrate often breaks during the production process. The construction of the cell including the thickness of the substrate is designed in view of the photoelectric conversion efficiency, the surface structure of the substrate or the like.
The silicon substrate which has been processed to have a thickness suitable for required cell characteristics is then passed through a number of washing steps. Thereafter, an uneven surface is formed on the light-receptive face of the substrate, to form a reflection-reducing surface structure. At this time, the reflection-reducing surface structure is formed in projections and depressions having a depth within a certain range in consideration of workability, conventionally. The depth here is the distance from the top of the projections to the bottom of the depressions, and also referred to as a projection-depression depth in some parts of this description.
For example, in the case of a solar cell using a monocrystalline silicon substrate, which is relatively easy to pattern, the surface structure is formed by etching. From the viewpoint of workability, it is important to shorten etching time as much as possible and to improve the accuracy of etching by reducing the area/volume of portions not to be etched. The projection-depression depth d2 (see FIG. 9(e)) of this solar cell is about 5 .mu.m to about 15 .mu.m.
The reflection-reducing surface structure, in the case of using a silicon substrate, is often formed to have projections and depressions which provide a so-called texture structure. The texture structure means a structure which may be formed in projections and depressions on a surface by utilizing differences in etch rate such that, when crystalline silicon is etched with a thin aqueous alkali solution, a (100) face in crystal orientation has a faster etch rate and a (111) face has a slower etch rate. In the texture structure, a projection is sometimes shaped in a pyramid (referred to as a normal pyramid) or a depression is sometimes formed by removing a pyramid shape (referred to as an inverted pyramid). The texture structure usually has a regular projection-depression structure which forms a lattice pattern as seen from a direction perpendicular to the surface and may be formed with use of a mask for etching which is made in a regular geometric pattern.
Such a texture structure may be formed, for example, by the following process.
In the case of using a monocrystalline silicon substrate, a mask material 13 to be used as a etching mask such as an oxide film is disposed on the substrate (see FIG. 9(b)).
Subsequently, the mask material 13 is patterned into a mask layer 15 by a photolithography and etching technique. At this time, if the monocrystalline silicon substrate has the (100) face, the mask layer 15 is as follows; transferred to the mask material 13 is a pattern having square windows 11 which have sides parallel to or perpendicular to a direction of an axis similar to a &lt;110&gt; axis of the crystal of the monocrystalline silicon and whose side length is about 10 .mu.m to about 20 .mu.m and frames 14 of 1 .mu.m width surrounding the windows 11, the square windows being continuously arranged at a pitch p1 of about 10 .mu.m to about 20 .mu.m. Thus a mask layer 15 having repeatedly arranged square windows (see FIG. 9(c)) is formed.
The monocrystalline silicon substrate 1 together with the mask layer 15 is immersed in a thin aqueous alkali solution to form projections and depressions (the reflection-reducing structure) 9 on the surface of the silicon substrate (see FIG. 9(d)). The thin aqueous alkali solution for etching may be, for example, an aqueous solution of potassium hydroxide of several percents which is heated to several ten degrees centigrade. By this process, the surface of the silicon substrate is etched at different etch rates depending on crystal orientation. As a result of using the mask material patterned into the aforesaid pattern, the surface of the silicon substrate is etched into a projection-depression surface structure which has a number of continuously arranged depressions 16 in the form of inverted pyramids exposing the (111) faces 19 which have the slowest etch rate.
This structure is generally called an inverted-pyramid texture structure since a lattice pattern can be seen from the direction perpendicular to the surface of the substrate. This texture structure may be produced to have a projection-depression depth d2 of 5 .mu.m to 15 .mu.m by setting a pitch p1 for arranging the squares of the mask pattern to 10 .mu.m to 20 .mu.m (see FIG. 9(e)).
After thus forming the monocrystalline silicon substrate 1 having the projections and depressions 9 of the reflection-reducing structure on the light-receptive side, a P-type diffusion layer 3 and an N-type high-concentration diffusion layer 2 are formed on the opposite side (non-light-receptive side) and on the light-receptive side, respectively (see FIG. 9(f)). An N-type-region-side insulating film 4 and a P.sup.+ -type-region-side insulating film 5 are formed. Thereafter, electrodes 6 and 7 are formed on the light-receptive side and the opposite side, and an anti-reflection film 8 is formed on the entire surface of the light-receptive face. Then, the substrate is cut into the final size, thereby to finish a monocrystalline silicon solar cell for space use (see FIGS. 9(g) and 7).
In space, an extremely large number of cosmic rays fly around. Space-use solar cells to be used in such environment are designed in view of the effect of cosmic rays. Solar cells exposed to radiation such as cosmic rays deteriorate in their characteristics including conversion efficiency, though the degree of deterioration varies depending on the types of solar cells. It can be said to be one of the most significant challenges in the designing of solar cells to realize the highest value for the minimum output (final output) during use in view of exposure to radiation in a use environment.
An artificial satellite in a stationary orbit is said to be exposed to about 1.times.10.sup.15 /cm.sup.2 of cosmic rays in terms of electron beams having an acceleration energy of 1 MeV for about ten years. The life of solar cells is often calculated using this level as a reference for a radiation resistant characteristic. Actually solar cells are exposed to this dose of electron beams for testing purposes, and a variety of data used for designing are computed from results of such exposure tests.
For example, the photoelectric conversion efficiency to an AM0 light source of common space-use silicon solar cells after exposure to 1 MeV electron beams at 1.times.10.sup.15 /cm.sup.2 is about 10%, that of high-efficiency silicon solar cells is about 12%, and that of GaAs-base solar cells is about 13%. As shown by these figures, GaAs materials may provide a more excellent radiation resistance for solar cells than silicon materials.
Silicon materials for electronic devices have a number of advantages which other materials do not have: the silicon materials can be generally used; their properties are well known; they are lower-priced than other semiconductor materials such as GaAs; they are stabile in quality; they are actually used in space environment. For these reasons, silicon solar cells are in widespread use as space-use solar cells. However GaAs-base solar cells, which have an advantage in the radiation resistance, often compete with the silicon solar cells.
If the radiation resistance of the silicon materials can be dramatically improved and their photoelectric conversion efficiency to the AM0 light source after exposure to 1 MeV electron beams at 1.times.10.sup.15 /cm.sup.2 can be raised to about 13% which is comparable to that of the GaAs materials without changing the properties of the silicon materials, the silicon materials will have the advantage over the GaAs materials in that the space-use silicon solar cells can have a radiation resistance equal to that of the GaAs solar cells while at the same time making the most of their benefits.
Approach for improving characteristics after exposure to radiation is generally divided into two directions. One is to improve initial characteristics before the exposure to radiation, and the other is to avoid effects of radiation. As to the former one, a lot of organizations have studied long since as one of the most important challenges in development of solar cells, and now it is becoming more difficult to find out a way to a great improvement of the present situation. As to the latter one, there lie a number of problems in applying theoretical means for improving characteristics to an actual production of solar cells and therefore it is not easy.
Generally, the thinner the substrate of a solar cell is, the less susceptible the solar cell is to the effect of radiation. For producing a thin solar cell, a thin substrate must be handled. However, in the case where a substrate of a crystalline semiconductor is used, as the substrate is thinner, the possibility that the substrate breaks during production of a solar cell becomes stronger and eventually the solar cell cannot be produced.
In conventional techniques, the possibly smallest thickness of the substrate is about 30 .mu.m for a solar cell having a simple construction requiring only a little patterning of the substrate and about 50 .mu.m for a high-efficiency solar cell whose substrate is subjected to a complicated patterning such as the reflection-reducing surface structure. As regards electrical output characteristics of these solar cells after exposure to radiation, the high-efficiency solar cell is more excellent even if its substrate has a thickness of 100 .mu.m or more, because the characteristics of these solar cells before the exposure to radiation are quite different. For example, the high-efficiency solar cell of 100 .mu.m thickness exhibits a conversion efficiency of about 12% to the AM0 light source after it is exposed to 1.times.10.sup.15 /cm.sup.2 of electron beams having an acceleration energy of 1 MeV, while the solar cell of a simple construction of 50 .mu.m thickness exhibits a conversion efficiency of about 11%.
With the trend to diversification and cost-reduction of satellites, there are demands for further improvement in performance. The conversion efficiency of the conventional high-efficiency solar cells is high for silicon solar cells, but it is demanded that the radiation resistance thereof be improved to a level equal to the GaAs-base solar cells and that the final output be enhanced. This has been considerably difficult.